LittleBits are awesome circuit kits that make playing around with electronics pretty simple and fun. If you'd rather make your own to see just how they work, Instructables user JackANDJude shows you how.

The basic premise of littleBits is that you get a box of modules that can be easily connected together and to real world items to form more complicated electronics. JackANDJude's DIY project is basically the same thing, but since you're building the components yourself, you end up learning a bit more about how they work. You'll need a soldering iron, hot glue, a ton of wire, and various other small parts, but you should be able to put everything together for pretty cheap. Head over to Instructables for the full guide and a rundown of everything you'll get.